Sometimes a material having one or more properties suited for use in a particular article of manufacture cannot be used because that material is not compatible with the process that is the most desirable for making the article. For example, in the context of polarized ophthalmic lenses, it is often desirable to attach a flexible polarizer to a rigid substrate to make a composite and then thermoform the composite to provide the curvature(s) necessary for the type of lens, e.g., plano, powered, multi-focal, etc., for which the composite will be used. After thermoforming, the composite may then be finished, e.g., by appropriately shaping its periphery to suit the finished lens and/or applying one or more optical coatings, such as a hardcoat, to the surfaces of the composite. Alternatively, the thermoformed composite may be used as pre-curved polarizing insert for a lens that, when finished, will include one or more additional optical layers attached to the insert. The resulting multilayer lens may then be finished in a manner similar to the manner mentioned, e.g., by appropriately shaping the periphery of the lens and/or applying one or more optical coatings to the surfaces of the lens.
Conventional polarizers used in ophthalmic lenses are often a sandwich of three thermoplastic layers. The middle layer is the polarizing layer, which frequently comprises a polyvinyl alcohol (PVA) layer containing either a hydrophilic dichroic dye or iodine that provides the polarizing property. The outer two layers are often made of the same material as each other, typically a cellulose-based polymer, such as cellulose aceto butyrate (CAB). Such polarizers provide very good polarizing performance, but they must not be heated to temperatures equal to or greater than their “minimum degradation temperature,” i.e., the temperature at which their performance/quality noticeably degrades due to physical changes caused by the elevated temperature. If these polarizers are heated to their minimum degradation temperature or higher, particularly for a sustained period, they will often become unsuitable for use because the degradation caused by the elevated temperature will reach or exceed an acceptable limit.
For example, a conventional CAB-PVA-CAB polarizer sandwich typically has a minimum degradation temperature in a range of about 95° C. to about 120° C., depending upon the particular formulation of the various layers. Relative to the CAB part of the polarizer, the amount and type of degradation generally depends on time and the internal constituents of the CAB. When such a polarizer is heated to or above its minimum degradation temperature, the CAB degrades, typically by yellowing in the first instance, followed by the development of air bubbles and/or blisters and increased stiffness. Similar degradation also occurs with cellulosetriacetate (CTA) but at slightly higher temperatures. Other degradation may also occur, depending upon the makeup of the polarizer. For example, depending upon the adhesive used bond the CAB layers to the PVA layer, the bond may separate in places and cause blisters among the layers. In another example wherein iodine is the polarizing substance incorporated into the PVA, the iodine's ability to provide a polarizing effect begins to irreversibly degrade at a temperature of about 100° C. (the minimum degradation temperature) by changing color. Moisture affects the amount and speed of the degradation; the higher the moisture content, the greater and faster is the degradation. Starting around 120° C., the polarizer begins to fade and lose polarizing efficiency.
Unfortunately, some of the most desirable thermoplastics for the substrate, based on their optical and durability properties, e.g., pure polycarbonates and some pure methacrylates, e.g., poly (methyl methacrylate) (PMMA), have glass transition temperatures that are higher than the minimum degradation temperatures of the polarizers. For example, a pure polycarbonate typically has a glass transition temperature in a range of about 135° C. to about 155° C., and a pure methacrylate typically has a glass transition temperature in a range of about 105° C. to about 110° C. Therefore, a manufacturing process that includes thermoforming the substrate and polarizer together with one another, which generally requires the substrate to be heated to its glass transition temperature or higher, would not be suitable because this temperature is higher than the minimum degradation temperature of the polarizer. Consequently, the polarizer would degrade, likely to an unacceptable extent. It is noted that in a thermoforming process that includes heating the article to a temperature lower than the glass transition temperature at issue and subjecting it to relatively large pressure is often not an acceptable alternative for ophthalmic lenses due to the residual stresses that this process imparts into article. Besides, even the lower temperatures used in this alternative are often higher than the minimum degradation temperature of the polarizer.